1. Field of the Invention
This invention relates to locating conductor-to-earth leakage paths through the insulation of buried conductors.
2. Description of the Prior Art
Telephone and electrical power lines are frequently buried in the earth for esthetic reasons and to reduce exposure to physical damage. These lines, however, do not always remain free of physical damage. The insulating material on a buried wire, for example, may be damaged during installation or as a result of earth shifting, rodent attack, or deterioration. When damage of this sort occurs, moisture in the earth often penetrates the insulation to provide a leakage path between the wire and the earth. On the other hand, when one or more wires are encased in a metallic sheath covered by insulating material, similar damage to this material often results in a leakage path between the metallic sheath and the earth. In either case, an electrical leakage path is provided between a metallic conductor and the earth which may have adverse effects on the intended function of the buried lines.
One technique for locating a fault of the above-described type applies an alternating test signal between the conductor and the earth and then uses a pair of earth probes to localize the fault by sensing the voltage gradient in the earth return path. With this technique, a phase reversal in the sensed voltage occurs as receiver probes transverse the fault. This is explained in detail in "Locating Cable Faults," by Charles A. Maloney, pp. 380-394 in I.E.E.E. Transactions on Industry Applications, Vol. 1A-9, No. 4, July/August 1973.
The above-mentioned technique has been utilized in test apparatus which applies a pulsed DC signal to a conductor under test and then detects the magnitude and polarity of probe-sensed pulses. It is also used in apparatus disclosed in U.S. Pat. No. 3,299,351, issued Jan. 17, 1967, to D. L. Williams. Williams' driving signal comprises a test signal and a second signal at one-half the frequency of the test signal. His receiver subsequently detects the test and second signals and utilizes the second signal to develop a reference signal for use in determining the relative phase of the detected test signal. Finally, the above-mentioned technique is utilized in method and apparatus disclosed in copending patent application Ser. No. 603,278, filed on Aug. 11, 1975, by T. V. Lathrop, now U.S. Pat. No. 3,991,363, and assigned to the same assignee as the present application. Lathrop's driving signal comprises a test signal and a second signal in the form of a carrier wave modulated by the test signal. At the receiver, the test and modulated signals are detected, the reference signal is derived from the detected modulated carrier wave, and the detected test signal is phase compared with the reference signal.